1. Technical Field
This invention relates to the general field of energy beam lithography and, more particularly, to prediction and correction of the lithography process to compensate for proximity heating of the resist and, most particularly, to correction of proximity heating occurring during resist exposure by means of vector scanning or stencil exposure.
2. Description of Related Art
The fabrication of integrated circuits (xe2x80x9cICsxe2x80x9d) requires ever more accurate methods for creating patterns on a wafer substrate, typically involving exposure of a resist-coated wafer to a pattern of energy in the form of electromagnetic radiation, electrons or other particle beams. xe2x80x9cPositive resistsxe2x80x9d require exposure of the resist to energy in those areas in which resist removal is desired. xe2x80x9cNegative resistsxe2x80x9d require exposure of the resist to energy in those areas in which resist retention is desired. Both positive and negative resists are commercially useful. Exposure to a pattern of energy may occur through a mask having a pattern of energy-transmissive regions therein, typically fully transmissive or fully opaque although xe2x80x9cgrayxe2x80x9d regions are not inherently excluded. xe2x80x9cPhotolithographyxe2x80x9d commonly denotes the exposure of a resist-coated substrate through a patterned mask by means of electromagnetic radiation.
Another general method of writing patterns on a resist makes use of a beam of energy directed only to those regions of the resist-coated surface requiring exposure without screening by an intervening mask. A suitable beam steering mechanism is typically employed along with suitable controls to insure that only the regions of the surface requiring exposure are contacted by the incident beam. The beam may be electrons, ions, neutral particles, collimated laser light or other electromagnetic radiation. However, to be definite in our discussion, we will emphasize the example of a beam of electrons impacting the resist-coated substrate (e-beam lithography), not excluding thereby other forms lithography by means of directed energy beams. The control mechanism may be a simple on-off control to expose the pattern pixel by pixel. The control mechanism may be more complex, controlling the beam intensity, shape, dwell time or other beam parameters, the control of which leads to precise patterning.
In addition to direct beam writing and lithography through a mask, circuit components (e.g. a memory cell) may be exposed in a single flash through a xe2x80x9cstencil.xe2x80x9d Stenciling in this manner may make use of either electromagnetic radiation or particle beams, and numerous uses of the same and different stencils may be necessary to fully pattern the entire surface of the wafer.
Direct beam writing of patterns onto a resist-coated surface is the method presently preferred for creating the masks used in photolithography, but direct writing offers other advantages as well. Among these other advantages of direct beam writing are the avoidance of complications of alignment and registration of the mask with the substrate and the possibility of creating more precise patterns with the use of accurately focused beams. One disadvantage of direct beam patterning in comparison with photolithography is the relatively smaller throughput possible with direct beam writing.
Considering e-beam lithography by way of example and not limitation, the presently employed writing techniques may be classified into general categories as vector scan, raster scan or stenciling. Vector scan typically directs the beam while off to a region of the substrate requiring exposure, then exposes a contiguous region of the substrate to the energy of the beam before moving to another region for exposure. Simply stated, vector scanning xe2x80x9cpaintsxe2x80x9d or xe2x80x9ctilesxe2x80x9d a region of the substrate with beam energy before moving on to expose another region. Most conveniently, beam direction, scanning trajectories, flash size, shape and/or intensity are under computer control, defining the pattern to be written.
Raster scanning directs the beam to all regions of the substrate no matter what pattern requires exposure and adjusts the beam intensity at each point scanned to effect the correct pattern of exposure. The simplest beam control during raster scanning entails having the beam on or off as each pixel is scanned. However, adjustment of beam intensity to numerous levels between full-on and full-off (gray scales) is also feasible in some raster scanning procedures. Stenciling combines use of one or more masks for patterning a single circuit component (such as a memory cell), with a flash exposure of the entire stencil.
Precise exposure of resist requires a detailed understanding of the sensitivity of the resist to e-beam exposure. The exposure of resist to an e-beam, called the dose, is typically measured in microcoulombs per square centimeter (xcexcC/cm2). The sensitivity of the resist means the electron dose (in xcexcC/cm2) necessary to create the desired pattern in the resist upon development. This sensitivity is a function of the resist composition, the energy of the incident electron beam, the temperature of the resist, the resist development process and other factors as well. The changes of resist sensitivity with its temperature at the time writing occurs is a particular concern of the present invention.
It is helpful to emphasize that exposure of the resist by e-beam impact and heating of the resist are two conceptually distinct phenomena. The chemical activity of the resist leading to its useful lithographic properties is initiated by e-beam impact. The efficacy of electrons in causing this chemical activity is defined as the sensitivity of the resist. The sensitivity of the resist to e-beam impact depends in turn on many factors including the temperature of the resist at the time it is exposed. Thus, changing the temperature of the resist changes its sensitivity which may require changing the dose of electrons, typically by changing the dwell time or beam current (or both) in order to achieve proper exposure. Failing to take into account changes in resist sensitivity with temperature may lead to overexposure of the resist, exposure of the resist in regions not intended to be fully exposed, and less precise patterns. Pattern xe2x80x9cbloomingxe2x80x9d is the undesired result.
Heating of the resist occurs in two ways: 1) As an inherent adjunct effect to the impact by electrons intentionally directed onto the resist for exposure. This heating is always present in e-beam lithography and is taken into account when the resist is calibrated to specify the correct exposure dose. 2) In high voltage lithography, most of the electron beam energy passes through the resist and the underlying mask layer (typically very thin) and penetrates the substrate where most of the beam energy is deposited. (An exception occurs when thin substrates are used, typically in the manufacture of X-ray masks, where the substrate is itself a film so thin that most of the beam energy passes through without significant diminution.) Electron diffusion in a thick substrate deposits the heat from a single e-bean flash in the substrate, typically in a volume 10 or more microns (xcexc or micrometers) in lateral extent (perpendicular to the e-beam direction). Subsequent thermal conduction transports a portion of this heat to the substrate surface where it heats the resist in a zone that may be tens of microns in lateral extent a few microseconds following the flash, increasing to a millimeter across after several milliseconds. (Exact numbers will depend on beam energy, the composition of the substrate and its thermal properties). Thereafter the heat has diffused so much as to have no significant effect on resist temperature or, consequently, on resist sensitivity. It is this second type of heating that this invention addresses and denotes as xe2x80x9cproximity heating.xe2x80x9d Such proximity heating depends on the previously written pattern and the time history of the pattern writing. This variability makes proximity heating particularly challenging to estimate in designing a process for high accuracy e-beam writing.
xe2x80x9cProximity heatingxe2x80x9d as used herein is not to be confused with the xe2x80x9cproximity effectxe2x80x9d related to the chemical effects of scattered electrons in the resist. Electrons in a beam passing through matter will from time to time encounter atomic nuclei or orbital electrons and undergo deflection from their line of travel (and/or scattering of orbital electrons into the surrounding medium), with or without loss of energy in the deflecting collision. Thus, scattered electrons within the resist may lead to exposure away from the desired exposure zone. The xe2x80x9cproximity effectxe2x80x9d relates to the chemical effect of these scattered electrons in exposing the resist, perhaps relatively far from the intended exposure zone at which the e-beam is directed. Backscattered electrons scattered from layers below the resist may re-enter the resist and also produce deleterious exposure. Many approaches have been suggested to ameliorate the effects of these scattered electrons as they lead to unwanted exposure of the resist, including that of Veneklasen et. al. (U.S. Pat. No. 5,847,959). Bohlen et. al. (U.S. Pat. Nos. 4,426,584 and 4,504,558) suggest a second exposure to the incident e-beam designed to correct for dosage losses or (for e-beam exposure through a mask) the use of two complimentary masks. Several ways to correct for the electron beam dosage have been suggested, including the work of Watson (U.S. Pat. No. 5,736,281), Ashton et. al. (U.S. Pat. No. 5,051,598), Owen et. al. (U.S. Pat. No. 5,254,438), and Chung et. al. (U.S. Pat. No. 5,432,714). In all cases, however, the focus of this prior work is to prevent or reduce the chemical effect of scattered electrons in causing undesired exposure of the resist. In contrast, the present invention relates to the thermal effect of both incident electrons and scattered electrons as they heat the target indirectly by conduction of heat deposited elsewhere, and the changes in resist sensitivity caused by this heating.
Proximity heating has been the subject of several calculations and measurements. Ralph et. al. describe methods for computing proximity heating by numerical integration of diffusion equations in xe2x80x9cProceedings of the Symposium on Electron and Ion Beam Science and Technology, Tenth International Conferencexe2x80x9d, p. 219-2330 (1983). Babin et. al. also describe methods for the numerical simulation of proximity heating and the comparison of such calculations with measured values. SPIE, Vol. 3048, p. 368-373 (1997) and J. Vac Sci Technol. B Vol. 16, pp. 3241-3247 (1998). Additional calculations of proximity heating and comparison with measured values have been reported by Yasuda et. al. in J. Vac Sci Technol. B Vol. 12, pp. 1362-1366 (1994).
Calculations of proximity heating may be based upon a numerical solution of the appropriate diffusion (partial differential) equations.
(c∂/∂txe2x88x92xcexa∇2)T(r,t)=Q(r,t)xe2x80x83xe2x80x83Eq.(1)
where T is the temperature, r is the position as a vector in 3-space, t is time, c the volumetric heat capacity and xcexa the thermal conductivity of the substrate. ∇2 is the Laplacian operator. Q(r,t) describes the space and time distribution of sources of heat (which may be negative as heat sinks). For most cases of practical interest, c and xcexa are substantially independent of temperature, resulting in Eq (1) being linear for thermal diffusion in the absence of sources (Q=0). That is, if T1(r, t) and T2(r, t) are each solutions of Eq. (1) for sources Q1 and Q2 respectively, then so T1(r, t)+T2(r, t) is a solution of Eq. (1) for source Q1+Q2.
In principle, an accurate solution for the resist temperature may be obtained by a two-step process for the solution of Eq. (1). The first step is to obtain Q(r, t), typically by means of a Monte Carlo simulation of electron impact onto the substrate. Equations describing the motion of an electron as it impinges upon and interacts with the substrate are numerically integrated to ascertain the path and energy deposition for a typical electron. This is repeated many times over a statistical sample of impinging electrons to obtain the rate of energy deposition in the substrate as a function of position and of time. The statistical sample is chosen to simulate the voltage, intensity, beam shape, etc. for a single flash. Commercial software is available for solving the thermal diffusion equation, Eq. (1) for the heat input thus obtained, leading to accurate results of resist temperatures resulting from a single flash, within the numerical precision of the calculations. ANSYS is one commercially available software package making use of direct numerical integration for solving the thermal diffusion equations. TEMPTATION is another commercially available software package making use of Green""s function procedures for solving the thermal diffusion equations for the Monte Carlo derived heat sources. Equivalent results are expected for either approach.
In principle, the above technique of flash-by-flash Monte Carlo simulation of heat deposition followed by numerical solution of the diffusion equation may be repeated for each flash in the pattern being written. The linearity of Eq. (1) permits these many solutions to be summed, yielding the temperature at any point in the resist. More particularly, the temperature at the point of the resist presently being written by the electron beam is the primary concern in order to insure correct exposure of the resist and accurate patterns. However, direct solution of the diffusion equation by the above techniques is much too slow to allow correction of the writing process for proximity heating to occur in real-time. Faster numerical procedures are necessary. In deriving useful approximations for such proximity heating, raster scanning offers some simplifications deriving from the predictability of the scanned path. That is, every point (pixel) in the previously scanned pattern (whether or not actually written) has a known relationship in space and time to the point presently being written. Thus, raster scanning relates the location and the time of writing of all pixels in the pattern in a simple manner that permits simplifications in the determination of proximity heating. These simplifications have been utilized by Veneklasen et. al. (U.S. Pat. No. 5,847,959) and Innes et. al. (U.S. patent application Ser. No. 09/343,960) to simplify the determination of proximity heating during raster scan patterning such that real-time correction of the beam during writing is feasible.
However, vector scanning paints or xe2x80x9ctilesxe2x80x9d various areas of the substrate in a largely unpredictable manner. Thus, unlike raster scanning xe2x80x9cwhenxe2x80x9d in the past a point on the substrate was written provides almost no information about xe2x80x9cwherexe2x80x9d on the substrate it was written, with the caveat that during the process of tiling, the writing tends to cluster in a contiguous region. As described elsewhere herein, this tendency to cluster in vector scanning is utilized to accelerate the determination of proximity heating according to the present invention, but is not an inherent limitation in the scope of applicability of the present invention. Conversely, clustered writing during vector scan patterning tends to exacerbate proximity heating, leading to temperature increases exceeding those of raster scanning by perhaps by a factor of 5-10.
The present invention relates to procedures for approximating the solution of the full thermal diffusion problem particularly appropriate for vector scanning or stenciling. It is shown below that a substantial reduction in the computer time necessary to predict proximity heating is achieved, permitting correction of the writing process to occur in compensation for this heating. More accurate patterning is the expected result.
Precise writing of patterns in a resist requires precise exposure of the resist which, in turn, requires precise knowledge of the sensitivity of the resist to e-beam impact. Resist sensitivity depends upon several factors including the temperature of the resist at the time of writing. Thus, the present invention relates to methods and procedures for determining resist temperature during processing and adjusting process parameters, including reducing the dwell time or beam current, to compensate for increased resist sensitivity. Typically, the resist temperature rise predicted by the present invention for the point of writing will be multiplied by a factor relating to the temperature sensitivity of the resist. The result is a correction applied to the dwell time or beam current to provide more accurate resist exposure. The correction will typically be a multiplicative factor less than 1 by which the energy deposited by the beam is to be adjusted to correct for proximity heating at the point of writing. It is envisioned that e-beam dwell time or e-beam current of each spot or xe2x80x9cflashxe2x80x9d may be adjusted. Pattern blooming is thereby reduced.
The present invention relates to methods of predicting proximity heating in real-time as the writing proceeds enabling beam compensation to be performed in real-time. Particular attention is given to vector scanning in which the pattern of writing is largely unpredictable, but the writing tends to cluster into cells. A library of standard cells is constructed. As writing of the pattern proceeds, the individual flashes are agglomerated into cells that are compared with standard cells to determine proximity heating in the resist as a function of the distance of the previously written cells from the point of present writing, and the elapsed time since writing a previously written cell. Further agglomeration of cells into super-cells, super-super-cells, etc. is also included within the scope of the present invention. An advantage of the present technique relates to its applicability to vector scanning and to stenciling techniques for writing patterns on the wafer surface, not dependent on foreknowledge of a particular pattern of writing, as is the case for raster or serpentine scanning. Thus, a generalization over the prior art is achieved. Advantages of the present invention include the prevention or mitigation of pattern blooming as incident electrons heat the resist and broaden the region of exposure applicable to vector or stencil writing of patterns.